Federal Regulations and the general requirements for safety in highway transportation have created the need for systems which will not only reduce the stopping distance of a rubber-tired vehicle, but also will prevent loss of directional control during maximum stopping.
It has long been known that a rubber tire has maximum tractive force or "grip" on the road when it is pushed beyond the state of simple static traction, but not so far as to lose all its "gearing" with the road surface. This range of maximum traction occurs when the tire angular speed is below the corresponding linear speed of the vehicle, i.e., when there is some degree of wheel slip. On most surfaces, it has been found that maximum tractive forces occur when the tire angular speed is at least 10 percent lower than the angular speed at which the tire would be in synchronization with the linear vehicle speed. Therefore, on any given road surface, the shortest stop possible can only be made if this condition is achieved. This condition, the wheel rotating below synchronization speed, is very difficult to obtain even by a very experienced test driver. Road friction variations, vehicle loading and brake sensitivity and stability are several of the major reasons why this condition is so difficult to obtain. Therefore, most drivers brake in such a manner that the vehicle wheels are either synchronized to vehicle speed or completely locked. Both these situations may result in a straightline stop, but there are exceptions. Generally, braked wheels that are synchronously rotating throughout the entire stop will give straightline stops. Theoretically, locked wheels should also give straightline stops, but frequently do not in actual practice because brakes do not always lock-up at the same time. The small initial angular impulse resulting from non-simultaneous lock-up starts the vehicle rotating as it slides. As the center of gravity shifts further and further off center, inertial forces continue to rotate the vehicle. A rotational deviation of approximately 20.degree. between the vehicle centerline and vehicle direction vector makes it almost impossible to regain control. Therefore, maximum controllability can only be achieved with rolling wheels.
These objectives are approximated using systems which sense an incipient wheel slid and thereupon momentarily reduce brake pressure until the wheel spins up to rolling speed; then reapply brake pressure until the incipient skid is again sensed. This brake pumping is automatically continued during the stop; the driver merely keeping firm pressure on the brake pedal. A typical wheel slip control system of this type is described in U.S. Pat. No. 3,857,760 entitled "Wheel Slip Control System For Automotive Vehicles And The Like", granted Aug. 6, 1974 in the name of Joseph E. Fleagle and assigned to the Wagner Electric Corporation, assignee of the present invention.
The wheel slip control systems for automotive vehicles, described in preceding paragraphs, depend on accurate monitoring of wheel speed to provide the basic input with which to calculate whether wheel slipping is occurring, when to apply correction to the braking pressure, and when to stop applying correction. One of the most convenient sensors from a production and maintenance standpoint consists of a stationary sensor retained in intimate contact with a rotating slotted or waffle-like surfaced tone wheel. The tone wheel consists of alternate ferrous metal bars and openings or peaks and valleys such that, when the bars and openings are moved past the sensor, the varying magnetic path seen by the sensor causes an electric signal to be induced in the sensing coil. The frequency of the signal in the sensing coil is proportional to the speed of the wheel as follows: EQU F = RPM/60 .times. K
where:
F = frequency (cycles per second) PA1 RPM = wheel revolution per minute PA1 K = number of slots in tone wheel
Adequate signal amplitude requires that close proximity be maintained between sensor and tone wheel. The required close proximity is usually achieved, beginning with the vehicle wheel hub removed from the vehicle, by sliding the sensor outward in its holding fixture, then installing and tightening the vehicle wheel hub, containing the tone wheel, on the axle. In the process of installing the vehicle wheel hub, the tone wheel is brought into contact with the sensor and presses the sensor back into its holding fixture as the wheel hub is tightened in place. When the vehicle wheel hub is fully installed, the tone wheel and sensor are in intimate rubbing contact. During subsequent operation of the vehicle, the face of the sensor becomes ground off by rubbing contact until a final close non-rubbing, proximity is achieved.
Sensors have heretofore been slidably retained in their holding fixtures using resilient elastomeric bushings. When the sensor is pushed back into its holding fixture during mounting of the wheel hub, the resilient bushing holds the sensor in place but also provides a spring-back force. As the faces of the sensor and tone wheel are ground away through rubbing contact, the spring-back force continues to press the face of the sensor against the tone wheel. As a consequence, the face of the sensor continues to lose material until the spring-back force of the resilient bushing is exhausted. This results in excessive wear on the sensor.
In addition, available elastomeric materials tend to harden and seize on the barrel of the wheel speed sensor after extended use. When the elastomeric bushing is so severely seized that the wheel speed sensor cannot be freed from it, no maintenance adjustment of sensor and tone wheel proximity is possible. At times, although the wheel speed sensor can be freed from the hardened elastomeric bushing, the hardened bushing is thereafter unable to adequately hold the wheel speed sensor.